Understanding SQL Server Stored Procedures and C# Interoperability

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In this article, we will explore the intricacies of SQL Server stored procedures and their interaction with C#. Specifically, we will delve into the issue of updating a varchar field in the first character only.

Introduction to SQL Server Stored Procedures

A stored procedure is a precompiled set of SQL statements that can be executed repeatedly without having to recompile them every time. In this article, we will focus on two main aspects of stored procedures:

Creation: How to create a stored procedure in SQL Server
Execution: How to execute a stored procedure using C#

Creating a Stored Procedure

To create a stored procedure in SQL Server, you can use the CREATE PROCEDURE statement. For example:

USE [testDatabase]
GO

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

ALTER PROCEDURE [dbo].[deepstoredProc] 
    @recid int, 
    @jsonstr nvarchar(max), 
    @text nvarchar(max)
AS
BEGIN
    UPDATE table1 
    SET transcript = @text, 
        field1 = 'Y' 
    WHERE recordingid = @recid

    INSERT INTO othertable  
    VALUES (@recid, '', 'textvalue1', COMPRESS(@jsonstr), 'en')

    INSERT INTO othertable  
    VALUES (@recid, '', 'textvalue2', COMPRESS(@text), 'en')
END

Executing a Stored Procedure using C#

To execute a stored procedure in C#, you can use the SqlCommand class. Here is an example of how to call the above stored procedure:

try
{
    using (var conn = new SqlConnection(connectionString))
    using (var command = new SqlCommand("deepstoredProc", conn)
               {
                   CommandType = CommandType.StoredProcedure
               })
    {
        command.Parameters.Add("@recid", SqlDbType.Int).Value = Int32.Parse(recid);
        command.Parameters.Add("@jsonstr", SqlDbType.VarChar, -1).Value = jsonstr;
        command.Parameters.Add("@text", SqlDbType.VarChar, -1).Value = txt;

        conn.Open();
        command.ExecuteNonQuery();
    }
}
catch (Exception e)
{
    WriteToSysLog("Exception execSP() " + e.Message);
}
finally
{
}

The Problem: Updating a varchar field in the First Character Only

In this example, we have a stored procedure that updates two tables (table1 and othertable) based on the input parameters. However, when calling the stored procedure using C#, only the first character of the @text value is updated for table1. This issue arises because of how SQL Server handles varchar values.

How SQL Server Handles Varchar Values

In SQL Server, a varchar field can store characters from 0 to its maximum length. When you update a varchar field, SQL Server does not automatically truncate the new value to the desired character count. Instead, it stores the entire new value in the database, and then returns only the first length characters when requested.

For example, if we have a varchar field with a length of 5 and we update its value to 'Hello', the database will store the following:

0x00: The null byte (indicating an empty string)
0x48: The first character ‘H’
0x65: The second character ’e'
0x6c: The third character ’l'
0x6f: The fourth character ‘o’

When we retrieve the updated value, SQL Server will return only these 4 characters ('Hello').

Solution: Truncating the Updated Value

To solve this problem, you can use the SUBSTRING() function in SQL Server to truncate the updated value to the desired length. Here is an example of how to modify the stored procedure:

ALTER PROCEDURE [dbo].[deepstoredProc] 
    @recid int, 
    @jsonstr nvarchar(max), 
    @text nvarchar(max)
AS
BEGIN
    UPDATE table1 
    SET transcript = SUBSTRING(@text, 1, 1) + ' ' + SUBSTRING(@text, 2, LEN(@text)),
        field1 = 'Y' 
    WHERE recordingid = @recid

    INSERT INTO othertable  
    VALUES (@recid, '', 'textvalue1', COMPRESS(@jsonstr), 'en')

    INSERT INTO othertable  
    VALUES (@recid, '', 'textvalue2', COMPRESS(@text), 'en')
END

In this modified version of the stored procedure, we use SUBSTRING() to extract only the first character ('H') and last character ('o') from the updated @text value. We then concatenate these two characters with a space in between to form the desired output.

Solution: Using C# to Truncate the Updated Value

Alternatively, you can also modify the C# code to truncate the updated value before executing it as a stored procedure:

try
{
    using (var conn = new SqlConnection(connectionString))
    using (var command = new SqlCommand("deepstoredProc", conn)
               {
                   CommandType = CommandType.StoredProcedure
               })
    {
        command.Parameters.Add("@recid", SqlDbType.Int).Value = Int32.Parse(recid);
        command.Parameters.Add("@jsonstr", SqlDbType.VarChar, -1).Value = jsonstr;
        command.Parameters.Add("@text", SqlDbType.VarChar, -1).Value = txt;

        conn.Open();
        var text = (string)command.Parameters["@text"].Value;
        var truncatedText = text.Substring(0, 1); // Truncate the updated value to only the first character

        command.Parameters["@text"].Value = truncatedText;
        command.ExecuteNonQuery();
    }
}
catch (Exception e)
{
    WriteToSysLog("Exception execSP() " + e.Message);
}
finally
{
}

In this modified version of the C# code, we use the Substring() method to extract only the first character from the updated @text value. We then assign this truncated value back to the original parameter.

Conclusion

SQL Server stored procedures can be a powerful tool for managing complex data operations, but they can also present challenges when working with varchar fields in C#. By understanding how SQL Server handles varchar values and using techniques such as truncating the updated value or modifying the stored procedure code, we can overcome these challenges and write efficient and effective stored procedures.

Last modified on 2024-03-18